1. Field of the Invention
This invention relates to a method of selectively etching a substrate.
This application is the US national phase of international application PCT/GB99/02331, filed in English on Jul. 20, 1999 which designated the US. PCT/GB99/02331 claims priority to GB Application No. 9816168.0 filed Jul. 25, 1998. The entire contents of these application are incorporated herein by reference.
2. Discussion of Prior Art
There are many methods of etching which are known. One example of substrate etching is for fabrication of porous silicon by photoelectrochemically etching patterned n-type silicon wafers. A standard KOH etchant is used to form pits with sharp points on the wafer surface and by back illumination of the silicon with above band-gap light, photogenerated holes concentrate at the points where localised dissolution takes place by anodic etching in HF.
Another example is etching of trench formations in n-type GaAs, also using HF as the etchant, for which the etch rate is improved by illumination with UV light. Etching of lithium niobate (LiNbO3) using ion driven techniques tends to be slow, so laser driven processes have been proposed where KF is applied to the surface of the LiNbO3 and a pulsed high power density laser beam locally melts the LiNbO3 which reacts with the KF to form a water soluble section which leaves a hole when rinsed. Increasing the power of the laser, increases the depth of the resultant hole.
All of the methods described aim to improve the rate of etching or depth of an etch by the use of illumination in combination with particular etchants or materials. This is an acceptable method for forming trenches, but less suitable for creating more complex structures in the substrate, which could only be done by setting up an etch pattern and relying on the areas that are not etched having the desired shape. Conventional etching of trenches also produces inclined sides with a reduced volume towards the bottom of the trench, so it is difficult to control the shape of a complex structure.
In SPIE Vol 2403 pp 409 to 412, Khudobenko describes fabrication of submicron gratings in p-GaAs where illumination of the sample surface diminishes the etching rate at that spot.
Applied Physics Letters, vol 71, No. 3, pp309 to 311 (21 Jul. 1997) by Ross et al. describes optically inducing periodic domain-inverted structures in bulk LiTaO3 by simultaneous application of an electric field and UV wavelength illumination.